The invention described herein may be manufactured, used, and licensed by the U.S. Government for governmental purposes without the payment of any royalties thereon.
1. Field of the Invention
This invention relates generally to uncooled infrared detectors and focal plane arrays, and more specifically to a method of fabricating an infrared. The invention relates more specifically to a method of fabricating a ferroelectric/pyroelectric detector. The invention relates even more specifically to a method of fabricating a detector using a rock salt structure material as a substrate and a crystallographically oriented bottom electrode material as a template for the growth of a crystallographically oriented ferroelectric/pyroelectric film.
2. Description of Related Art
Uncooled infrared thermal detectors have recently been made into large-size focal plane arrays (hereinafter xe2x80x9cFPAxe2x80x9d). For ferroelectric (xe2x80x9cFExe2x80x9d) and pyroelectric (xe2x80x9cPExe2x80x9d) infrared detectors, the detector structures are capacitors, in which the FE or PE (xe2x80x9cFE/PExe2x80x9d) thin film is disposed between top and bottom thin electrode layers. In one type of detector, the FE/PE thin film is disposed between a reflective bottom electrode and a semi-transparent top electrode. In this type of detector, the resonance of the absorption occurs within the detector structure itself. In a second type of detector, the FE/PE thin film is disposed between a transparent bottom electrode and a semi-transparent top electrode. In this type of detector, the resonance of the absorption occurs in the cavity between the detector structure and the reflecting layer of the readout integrated circuit (xe2x80x9cROICxe2x80x9d).
In conventional FE/PE detectors, the bottom electrode is grown on top of a sacrificial layer, such as, polyimide or amorphous or randomly oriented materials. The subsequent FE/PE thin films are not, therefore, crystallographically oriented. By providing oriented FE/PE materials, however, the temperature coefficient of resistance (xe2x80x9cTCRxe2x80x9d) in such detectors can be improved by a factor of as much as three.
For oriented growth of the FE/PE sensing materials, the electrode should provide a structure lattice match and be chemically compatible with the substance and the sacrificial layer. Further requirements include that the top and bottom electrode layers that sandwich the FE/PE material be semi-transparent, so that part of the light can pass through to allow multi-pass absorption, and that the sheet resistance of the electrode film be within a specific range of values in order to maximize the resonance absorption in the structure. Examples of lattice matching materials include xe2x80x9cnormalxe2x80x9d metals such as Pt (lattice constant xe2x80x9caxe2x80x9d=3.89 xc3x85) and Pd (3.92 xc3x85); conducting oxides such as La1xe2x88x92xSrxCoO3 (xe2x80x9cLSCOxe2x80x9d) (3.8-3.9 xc3x85); and colossal magneto-resistive (xe2x80x9cCMRxe2x80x9d) materials such as La1xe2x88x92xCaxMnO3 (3.8-3.9 xc3x85). As electrode materials, the normal metals, are less desirable than the semi-transparent conducting oxide and colossal magneto-resistive materials. Pt, for example, not only is not semi-transparent, but is highly reflective. In addition, with Pt, the formation of helices at the deposition temperatures required for growth of the crystalline FE/PE films causes further complications for the growth of oriented films.
Perovskite oxide materials are of interest for use as the FE/PE film due to their typically superior figure of merit, which translates into superior device performance. Many FE/PE materials with the perovskite oxide structure have an a and a b lattice constant of approximately 3.9 xc3x85. Thus, materials that provide a lattice match to the perovskite oxide structure can be used both as a template for growth of perovskite oxides such as FE/PE materials, and as conducting oxides such as LSCO and CMR materials for use as semi-transparent electrodes.
The use of CMR materials for uncooled infrared detectors is described in Goyal et al., A., xe2x80x9cMaterial Characteristics of Perovskite Manganese Oxide Thin Films for Bolometric Applications,xe2x80x9d Applied Physics Letters, Vol. 71 (17) (Oct. 27, 1997), pp. 2535-2537. CMR materials demonstrate an exceptionally large change in resistance with temperature as they transition from a ferromagnetic to a non-ferromagnetic phase. The transition temperature can be adjusted through appropriate selection of materials and process conditions. The results have demonstrated the feasibility of growing CMR thin films on perovskite oxide material substrates such as LaAlO3 and SrTiO3 with a resultant temperature coefficient of resistance of greater than 7%.
CMR materials have a perovskite crystal structure with a square base. The lattice constant a of the square base of a typical CMR material is approximately 3.8 to 3.9 xc3x85 depending on the material composition. As indicated above, CMR thin films have been successfully grown on perovskite oxide substrates such as LaAlO3 and SrTiO3, and exhibit a good crystal orientation and a high temperature coefficient of resistance. These perovskite oxide substrate materials are employed because of the correspondence of their crystal structure and lattice constant to those of CMR materials. For example, SrTiO3 has a cubic crystal structure with a lattice constant of 3.905 xc3x85, and LaAlO3 has a pseudo-cubic crystal structure with a lattice constant of 3.79 xc3x85. These properties facilitate the growth of a CMR material on LaAlO3 and SrTiO3 with a resultant high crystal orientation and quality. A disadvantage associated with use of these materials, however, is that both LaAlO3 and SrTiO3 are very difficult to remove once the detector array has been bonded to the ROIC.
Therefore, a general need exists to provide a method of fabricating an uncooled ferroelectric/pyroelectric infrared detector that includes a semi-transparent electrode material. A more specific need exists to provide a method of fabricating a detector having a crystallographically oriented ferroelectric/pyroelectric film. An even more specific need exists to provide such a method in which the substrate can be easily removed once fabrication of the detector has been completed.
It is an object of the present invention to provide a method of fabricating an uncooled ferroelectric/pyroelectric infrared detectors and arrays that comprises a semi-transparent electrode material.
It is a further object of the present invention to provide a method of fabricating detector arrays using lattice matched substrate materials and a crystallographically oriented bottom electrode material as a template for the growth of a crystallographically oriented ferroelectric/pyroelectric film.
It is a still further object of the present invention to provide a method in which the substrate can be easily removed once fabrication of the detector array has been completed.
Accordingly, in a first preferred embodiment, the present invention advantageously relates to a method of fabricating a ferroelectric/pyroelectric detector using a thin film transfer method comprising deposition of a rock salt structure material substrate on a lattice matched reflecting metal layer. The method comprises (a) depositing a lattice matched reflecting metal layer on a top surface of a circuit; (b) depositing a rock salt structure material substrate as a sacrificial layer on a deposition surface of the metal layer; (c) depositing a lattice matched and crystallographically oriented first electrode layer on a deposition surface of the rock salt substrate layer; (d) depositing a lattice matched and crystallographically oriented ferroelectric/pyroelectric film layer on a deposition surface of the first electrode layer; (e) depositing a second electrode layer on a deposition surface of the ferroelectric/pyroelectric film layer; and (f) removing the rock salt structure material sacrificial layer.
By using a rock salt structure material such as, for example, NaCl, LiF, NaF, KF, or KCl as the sacrificial layer, a crystallographically oriented bottom electrode material can be grown, which then serves as a template for the growth of a crystallographically oriented ferroelectric/pyroelectric film. The rock salt sacrificial layer can be removed using water, and the excess solution can then be removed by a method such as evaporation, triple-point, or freeze drying.
In a second preferred embodiment, the present invention relates to a method comprising fabricating a detector array assembly, inverting the assembly, and attaching the inverted assembly to the ROIC. This embodiment avoids temperature processing constraints associated with the ROIC, and thus facilitates the use of higher growth temperatures. The method comprises (a) depositing a first electrode layer on the rock salt substrate or buffered rock salt substrate, or a substrate buffered with rock salt; (b) depositing a lattice matched and crystallographically oriented ferroelectric/pyroelectric film layer on a deposition surface of the first electrode layer; (c) depositing a lattice matched and crystallographically oriented second electrode layer on a deposition surface of the ferroelectric/pyroelectric film layer to form a detector assembly; (d) inverting the detector assembly; (e) depositing a lattice matched reflecting metal layer on a top surface of a circuit; (f) attaching the inverted detector assembly to an attachment surface of the metal layer with a bonding material; and (g) removing the rock salt structure material substrate.
By employing the second preferred embodiment, the growth and processing temperatures can be higher than those employed when the electrode and FE/PE materials are grown in close proximity to the ROIC, thus yielding materials of higher crystal orientation and even higher quality.
Advantages associated with the embodiments of the present invention include the ability to fabricate a crystallographically oriented bottom electrode material using rock salt as a template for the growth of a crystallographically oriented ferroelectric/pyroelectric film. Furthermore, once the fabrication is complete, the substrate upon which the bottom electrode is deposited can be easily removed. Finally, by virtue of the crystallographically oriented ferroelectric/pyroelectric film, a significant improvement in the overall performance of the detector, and thus the focal plane array, is achieved.